1. Field of the Invention
The present invention relates to prepregs comprising fibres and thermosetting resin which are stacked to form a laminate and subsequently cured to form a composite material. More particularly, the present invention involves the use of thermoplastic veil or other light weight structured thermoplastic material in the laminate interleaves. The present invention also relates to improvements in the electromagnetic response of composite materials, particularly to providing improved resistance to damage caused by lightning strikes.
2. Description of Related Art
Composite materials have well-documented advantages over traditional construction materials, particularly in providing excellent mechanical properties at very low material densities. As a result, the use of such materials is becoming increasingly widespread and their fields of application range from “industrial” and “sports and leisure” to high performance aerospace components.
Prepregs, comprising a layer of fibre impregnated with resin such as epoxy resin, are widely used in the generation of such composite materials. Typically a number of plies of such prepregs are “laid-up” as desired and the resulting laminate is cured, typically by exposure to elevated temperatures, to produce a cured composite laminate.
However, although such cured materials have a number of clear benefits, it has long been known that they can suffer from poor impact resistance and be prone to delamination. This is particularly the case when epoxy resin systems are used, which are known to tend to produce cured systems with low toughness.
A widely employed method of improving the toughness of such arrangements is for the laminate of a plurality of prepreg fibre layers to be interleafed with resin layers. Commonly such resin interleaf layers also comprise a distribution of thermoplastic toughener particles. This arrangement has been shown to increase the toughness of the laminate without having a detrimental effect on other aspects of the laminate.
Laminates that have interleaf layers toughened with thermoplastic particles are typically cured under autoclave conditions, where the high temperatures, and more importantly the high pressures, are generally required to provide cured laminates that meet the particularly exacting mechanical specifications required for structural applications.
A widely used alternative to autoclave cure is the so-called vacuum bag or out-of-autoclave cure. This utilizes a vacuum and relies on atmospheric pressure to press down onto the laminate during cure. Although much more economical than autoclave curing, the maximum pressure applicable in out-of-autoclave curing is atmospheric pressure. Laminates that have interleaf layers toughened with thermoplastic particles have typically not been cured outside of an autoclave because curing at atmospheric pressure or below tends to produce cured laminates that have unacceptable mechanical properties for many structural applications including aerospace structural applications.
It would therefore be desirable to develop a prepreg which could be used to produce laminates that can be cured, either inside or outside of an autoclave, to provide composite parts that are sufficiently tough to be suitable for structural application including aerospace structural applications.
A common composite material is made up from a laminate of a plurality of prepreg fibre layers, e.g. carbon fibres, interleafed with resin layers. These resin layers are believed to provide a significant toughness improvement to the eventual cured laminate.
Although the carbon fibres have some electrical conductivity, the presence of the interleaf layers means that this is only exhibited in the composite in the plane of the laminate. The electrical conductivity in the direction orthogonal to the surface of the laminate, the so-called z-direction, is low.
This lack of conductivity in the z-direction is generally accepted to contribute to the vulnerability of composite laminates to electromagnetic hazards such as lightning strikes. A lightning strike can cause damage to the composite material which can be quite extensive, and could be catastrophic if occurring on an aircraft structure in flight. This is therefore a particular problem for aerospace structures made from such composite materials.
Additionally, composites for use in aerospace applications must meet exacting standards on mechanical properties. Thus, any improvements in conductivity must not impact negatively on mechanical properties.
A wide range of techniques and methods have been suggested in the prior art to provide lightning strike protection to such composite materials, typically involving the addition of conductive elements at the expense of increasing the weight of the composite material. In WO 2008/056123 significant improvements have been made in lightning strike resistance, without significantly increasing weight or affecting mechanical properties, by including metallic conductive particles in the resin interleaf layers so that they contact the adjacent fibre layers and create an electrical pathway in the z-direction. EP 2053078 A1 teaches a prepreg comprising conductive particles and thermoplastic particles.
The present invention aims to obviate or at least mitigate the above described problems and/or to provide improvements generally.